Antibiotics have proven effective in eliminating or at least greatly reducing the incidence of many diseases caused by bacteria because these compounds possess the necessary selectivity to attack bacterial cells while sparing human cells. Unfortunately, the widespread use of common antibiotics such as penicillin has selected for resistant strains that are no longer susceptible to these agents. Since resistance is appearing to even the most potent antibiotics, such as vancomycin, the development of new approaches in antimicrobial therapy is imperative. The discovery of naturally occurring antimicrobial peptides opened a new dimension for antibiotic development. Magainins, isolated from frog skin, are representative of the class of small linear cationic peptides that can kill both Gram-positive and Gram-negative bacteria by increasing the permeability of the plasma membrane at concentrations that do not induce hemolysis. PGLa, also isolated from frog skin, has greater antimicrobial activity than magainins while retaining low hemolytic activity. A common feature of these peptides is their capacity to form an amphipathic alpha-helix (with polar and nonpolar groups on opposite faces of the helix), a structural feature believed to be important in their function as antimicrobial agents. Numerous analogues with sequences derived from these peptides have been prepared and examined. In nearly all cases, the strategy employed in enhancing activity involved increasing the amphi-pathic alpha-helical character of the peptide. To date, no one has attempted a comprehensive study of the structure-function relationships of a family of closely related linear peptides with simple sequences designed to adopt different secondary structures. This approach will reveal the importance of different secondary structures with varying levels of amphipathic character in determining antimicrobial activity and selectivity between bacterial and mammalian membranes. We propose here to synthesize two families of peptides with varying capacity to form amphipathic alpha-helic and beta-sheet structures. Using these two families of peptides, we will investigate the following areas: 1) the relationship of secondary structure and amphipathic character of the peptides to antimicrobial activity; 2) the amount of peptide that must bind to the membrane in order to increase permeability; and 3) the role of the membrane lipid composition in determining susceptibility to peptide-induced increase in permeability. This project should facilitate the design of effective antimicrobial peptides that will augment the arsenal of available antibiotics in order to keep pace with the ever-increasing resistance of bacteria to the drugs in current use.